The present invention relates, in general, to electronic devices. More particularly, the present invention provides a transparent solar cell and optical filter formed with a Schottky barrier diode and method of its manufacture.
Solar energy provides many advantages over traditional energy sources. For example, energy from the sun is virtually unlimited and easily accessible throughout the world. It does not require the extraction of a natural resource from the ground to obtain the energy and it can be converted to electricity in a manner that is not harmful to the environment. Solar energy is available whenever the sun is shining and can be collected and stored for use when no light source is available. Therefore, if it can be harnessed economically, it provides an environmentally friendly source of energy that does not deplete or destroy precious natural resources. This is in stark contrast to the use of fossil fuels that are of limited supply and which cause environmental damage with both their use and extraction processes. The use of fossil fuel also requires a constant source of raw materials that may be difficult to obtain in many circumstances.
Many different applications benefit greatly from the use of solar energy. For example, buildings and automobiles with their broad surfaces that are exposed to the sun""s energy for much of the day can use that energy to provide some or all of their energy needs. Various solar cells have been developed using different fabrication techniques to take advantage of this energy source.
The inventor of the present invention has previously filed patent applications directed toward a particularly beneficial solar cell. That patent application describes a structure that includes a p-n junction diode. The p+ and nxe2x88x92 polycrystalline silicon structures making up the pin junction are formed using an Excimer laser. An advantage of using the Excimer laser is that it may form the polycrystalline silicon without destroying a low melting point substrate upon which the solar cell is fabricated.
The prior art also includes other types of solar cells with pin junctions. One type of solar cell is formed with crystalline silicon. For these solar cells, crystalline silicon is formed by melting silicon and drawing an ingot of crystalline silicon of the size desired. Alternatively, a ribbon of crystalline silicon can be pulled from molten silicon to form a crystalline silicon solar cell. A conductor is placed on either side of the crystalline silicon to form the solar cell. These processes use high temperatures and the solar cells are expensive to manufacture. Packaging is also difficult and expensive and creates a rigid structure. Their maximum size is limited by the manufacturing process. It is difficult to slice the resulting crystalline silicon thin enough to provide a transparent or flexible solar cell. However, these structures are very efficient (relative to other types of presently available commercial solar cells). As such, crystalline solar cells are used primarily for applications where efficiency is more important than cost and where the structures do not need to be flexible. For example, these are commonly used on satellites.
Another type of solar cell is formed with polycrystalline silicon. These may be formed as thin layers on wafers and can thus be made thinner than crystalline silicon solar cells. As is well known in the art, polycrystalline silicon can be formed by heating amorphous silicon and allowing it to cool. Typically, amorphous silicon begins to crystallize after it melts at temperatures greater than about 1400xc2x0 C. and begins to cool below that level. Because of these high temperatures, known processes can only use substrates with high melting points. These processes are not appropriate for substrates made of plastics or other materials that melt at lower temperatures. In the manufacture of flat panel displays, it is known to use lasers to form polycrystalline silicon thin film transistors (TFTs). Such use has not included the formation of P-N junctions or solar cells which presents its own set of challenges. Moreover, these manufacturing processes generally formed single transistors and were not used to form large sheets or areas of polycrystalline silicon. Further, lasers have been used in the manufacture of solar cells, but only as a tool to mechanically form (slice, pattern, etch, etc.) the solar cells.
Another type of solar cell has been formed using doped layers of amorphous silicon. These are not subject to some of the problems inherent in the previously described crystalline silicon or polycrystalline solar cells. First, amorphous silicon can be formed using low temperature processes. Thus, it can be formed on plastic and other flexible substrates. They can also be formed over large surfaces. Second, the processing techniques are less expensive. Nevertheless, amorphous solar cells introduce other significant limitations not found in crystalline silicon or polycrystalline silicon solar cells. For example, hydrogen is generally added during the manufacturing to increase the efficiency of the cell. Amorphous silicon solar cells tend however to lose this hydrogen over time, causing reduced efficiency and reduced usable life. Moreover, amorphous silicon solar cells are not transparent. Thus, they are not appropriate for many applications. For example, buildings and cars with solar cells can be unsightly, and the solar panels may block the view of the outdoors or access to outside light indoors. Also, portable electronics often place a premium on size and surface area. Some devices have displays that cover mostxe2x80x94if not allxe2x80x94of the exposed surface of the device. Therefore, it is often undesirable or impossible to mount a traditional amorphous silicon solar cell on the device.
Attempts have been made to solve this transparency problem by making transparent panels from existing solar cell processes. One method has been to take advantage of the xe2x80x9cwindow shade effectxe2x80x9d whereby solar cells are formed on a transparent substrate with gaps between adjacent solar cells. This allows some light to pass through to create a transparent effect. The larger the gaps, the more transparency the device has. A disadvantage of this technique is that much of the space is unused, therefore the efficiency of the device is less than it would be if all of the surface area was used for solar cells. Of course, devices of this type also suffer from the problems inherent to the type of cell used. For example, if based on amorphous silicon, these devices suffer from the hydrogen loss exhibited in other amorphous silicon devices.
Other work has been done at making transparent solar cells using materials other than silicon (for example, cadmium telluride (CdTe)). These cells suffer from the challenges inherit to using materials other than silicon.
Thus, a new solar cell and method of fabrication that will avoid these problems and is more efficient to manufacture is desirable.
The present invention provides improved devices such as transparent solar cells and optical filters. It also provides improved methods for forming those devices. In contrast with devices and methods previously disclosed by the present inventor, these improved devices and methods use fewer layers resulting in simpler, less expensive fabrication processes and resulting in simpler devices along with other beneficial results. Moreover, compared with other fabrication techniques, the present invention allows for the fabrication of devices that are transparent using existing fabrication equipment and processing steps, while allowing those processes to be done quickly. In some embodiments, the processes may be completed on low melting point substrates that would be destroyed using previously known techniques.
In a first embodiment of the present invention, a method is provided for fabricating a device. The method comprises forming a first conductive layer overlying a substrate, forming a first amorphous silicon layer overlying the first conductive layer and annealing the first amorphous silicon layer by applying thermal energy with a laser to convert amorphous silicon of the first amorphous silicon layer into polycrystalline silicon. A second conductive layer may be formed overlying the polycrystalline silicon. The methodology produces a Schottky barrier diode between the conductive layer and the polycrystalline silicon. The resulting device may be used as a solar cell or as an optical filter. Steps of the methodology may be repeated to create successive layers of conductors and polycrystalline silicon.
In another embodiment of the present invention, a method is provided for fabricating a transparent device. The method comprises forming a first conductive layer overlying a transparent substrate, forming a first amorphous silicon layer overlying the first conductive layer; and converting the first amorphous silicon layer into polycrystalline silicon by application of thermal energy while maintaining the transparent substrate at a temperature of less than 450xc2x0 C. An Excimer laser or similar device may be used for applying the thermal energy. Again, a second conductive layer may be formed overlying the polycrystalline silicon and alternating layers of polycrystalline silicon and conductive layers may be formed in subsequent layers.
In yet another embodiment of the present invention, a device that may be used for example as an optical filter or a solar cell is disclosed. The device comprises a substrate with a melting temperature of less than 450xc2x0 C., a first conductive layer overlying the substrate and a first polycrystalline film formed from a first amorphous silicon layer overlying the first conductive layer. Alternating conductive layers and polycrystalline film layers may be placed above those layers to increase the efficiency of the device.
A further understanding of the nature and advantages of the inventions presented herein may be realized by reference to the remaining portions of the specification and the attached drawings.